Eris (minor-planet designation136199 Eris) is the most massive and second-largest dwarf planet known in the Solar System. Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year. In September 2006 it was named after Eris, the Greek goddess of strife. Eris is the ninth most massive object directly orbiting the Sun, and the 16th most massive overall, because seven moons are more massive than all known dwarf planets. It is also the largest which has not yet been visited by a spacecraft. Eris was measured to be 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi) in diameter.[8] Eris' mass is about 0.27% of the Earth mass,[10][16] about 27% more than dwarf planet Pluto, although Pluto is slightly larger by volume.[17]

Because Eris appeared to be larger than Pluto, NASA initially described it as the Solar System's tenth planet. This, along with the prospect of other objects of similar size being discovered in the future, motivated the International Astronomical Union (IAU) to define the term planet for the first time. Under the IAU definition approved on August 24, 2006, Eris is a "dwarf planet", along with objects such as Pluto, Ceres, Haumea and Makemake,[22] thereby reducing the number of known planets in the Solar System to eight, the same as before Pluto's discovery in 1930. Observations of a stellar occultation by Eris in 2010, showed that its diameter was 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi), very slightly less than Pluto,[23][24] which was measured by New Horizons as 2,372 ± 4 kilometers (1,473.9 ± 2.5 mi) in July 2015.[25]

Routine observations were taken by the team on October 21, 2003, using the 1.2 m Samuel OschinSchmidt telescope at Palomar Observatory, California, but the image of Eris was not discovered at that point due to its very slow motion across the sky: The team's automatic image-searching software excluded all objects moving at less than 1.5 arcseconds per hour to reduce the number of false positives returned. When Sedna was discovered, it was moving at 1.75 arcsec/h, and in light of that the team reanalyzed their old data with a lower limit on the angular motion, sorting through the previously excluded images by eye. In January 2005, the re-analysis revealed Eris's slow motion against the background stars.

Follow-up observations were then carried out to make a preliminary determination of Eris's orbit, which allowed the object's distance to be estimated. The team had planned to delay announcing their discoveries of the bright objects Eris and Makemake until further observations and calculations were complete, but announced them both on July 29 when the discovery of another large TNO they had been tracking, Haumea, was controversially announced on July 27 by a different team in Spain.[2]

Precovery images of Eris have been identified back to September 3, 1954.[3]

More observations released in October 2005 revealed that Eris has a moon, later named Dysnomia. Observations of Dysnomia's orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be 7022166000000000000♠(1.66±0.02)×1022 kg,[10]6999270000000000000♠27%±2% greater than Pluto's.

Eris is named after the Greek goddess Eris (Greek Ἔρις), a personification of strife and discord.[27] The name proposed by the CalTech Team on September 6, 2006, and it was assigned on September 13, 2006,[28] following an unusually long period in which the object was known by the provisional designation2003 UB313, which was granted automatically by the IAU under their naming protocols for minor planets. The regular adjectival form of Eris is Eridian.

Due to uncertainty over whether the object would be classified as a planet or a minor planet, because different nomenclature procedures apply to these different classes of objects,[29] the decision on what to name the object had to wait until after the August 24, 2006, IAU ruling.[30] As a result, for a time the object became known to the wider public as Xena.

"Xena" was an informal name used internally by the discovery team. It was inspired by the title character of the television series Xena: Warrior Princess. The discovery team had reportedly saved the nickname "Xena" for the first body they discovered that was larger than Pluto. According to Brown,

We chose it since it started with an X (planet "X"), it sounds mythological (OK, so it's TV mythology, but Pluto is named after a cartoon, right?),[h] and (this part is actually true) we've been working to get more female deities out there (e.g.Sedna). Also, at the time, the TV show was still on TV, which shows you how long we've been searching![32]

"We assumed [that] a real name would come out fairly quickly, [but] the process got stalled", Mike Brown said in interview,

One reporter [Ken Chang][33] called me up from the New York Times who happened to have been a friend of mine from college, [and] I was a little less guarded with him than I am with the normal press. He asked me, "What's the name you guys proposed?" and I said, "Well, I'm not going to tell." And he said, "Well, what do you guys call it when you're just talking amongst yourselves?"... As far as I remember this was the only time I told anybody this in the press, and then it got everywhere, which I only sorta felt bad about—I kinda like the name.[34]

Artist's impression of the dwarf planet Eris and its large moon Dysnomia. This artistic representation is based on observations made at ESO's La Silla Observatory.[35]

According to science writer Govert Schilling, Brown initially wanted to call the object "Lila", after a concept in Hindu mythology that described the cosmos as the outcome of a game played by Brahma. The name was very similar to "Lilah", the name of Brown's newborn daughter. Brown was mindful of not making his name public before it had been officially accepted. He had done so with Sedna a year previously, and had been heavily criticized. However, no objection was raised to the Sedna name other than the breach of protocol, and no competing names were suggested for Sedna.[36]

He listed the address of his personal web page announcing the discovery as /~mbrown/planetlila and in the chaos following the controversy over the discovery of Haumea, forgot to change it. Rather than needlessly anger more of his fellow astronomers, he simply said that the webpage had been named for his daughter and dropped "Lila" from consideration.[37]

Brown had also speculated that Persephone, the wife of the god Pluto, would be a good name for the object.[2] The name had been used several times in science fiction,[38] and was popular with the public, having handily won a poll conducted by New Scientist magazine ("Xena", despite only being a nickname, came fourth).[39] This was not possible once the object was classified as a dwarf planet, because there is already an asteroid with that name, 399 Persephone.[2]

With the dispute resolved, the discovery team proposed Eris on September 6, 2006. On September 13, 2006 this name was accepted as the official name by the IAU.[40][41] Brown decided that, because the object had been considered a planet for so long, it deserved a name from Greek or Roman mythology, like the other planets. The asteroids had taken the vast majority of Graeco-Roman names. Eris, whom Brown described as his favorite goddess, had fortunately escaped inclusion.[34] The name in part reflects the discord in the astronomical community caused by the debate over the object's (and Pluto's) classification.

Eris is a trans-Neptuniandwarf planet (plutoid).[42] Its orbital characteristics more specifically categorize it as a scattered-disk object (SDO), or a TNO that has been "scattered" from the Kuiper belt into more-distant and unusual orbits following gravitational interactions with Neptune as the Solar System was forming. Although its high orbital inclination is unusual among the known SDOs, theoretical models suggest that objects that were originally near the inner edge of the Kuiper belt were scattered into orbits with higher inclinations than objects from the outer belt.[43] Inner-belt objects are expected to be generally more massive than outer-belt objects, and so astronomers expect to discover more large objects like Eris in high-inclination orbits, which have traditionally been neglected.

Because Eris was initially thought to be larger than Pluto, it was described as the "tenth planet" by NASA and in media reports of its discovery.[44] In response to the uncertainty over its status, and because of ongoing debate over whether Pluto should be classified as a planet, the IAU delegated a group of astronomers to develop a sufficiently precise definition of the term planet to decide the issue. This was announced as the IAU's Definition of a Planet in the Solar System, adopted on August 24, 2006. At this time, both Eris and Pluto were classified as dwarf planets, a category distinct from the new definition of planet.[45] Brown has since stated his approval of this classification.[46] The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.[30]

Seen from earth, Eris makes small loops in the sky through the constellation of Cetus

The orbit of Eris (blue) compared to those of Saturn, Uranus, Neptune, and Pluto (white/gray). The arcs below the ecliptic are plotted in darker colors, and the red dot is the Sun. The diagram on the left is a polar view whereas the diagrams on the right are different views from the ecliptic.

Eris's orbit is highly eccentric, and brings Eris to within 37.9 AU of the Sun, a typical perihelion for scattered objects. This is within the orbit of Pluto, but still safe from direct interaction with Neptune (29.8–30.4 AU). Pluto, on the other hand, like other plutinos, follows a less inclined and less eccentric orbit and, protected by orbital resonance, can cross Neptune's orbit. It is possible that Eris is in a 17:5 resonance with Neptune, though further observations will be required to check that hypothesis.[49] Unlike the eight planets, whose orbits all lie roughly in the same plane as the Earth's, Eris's orbit is highly inclined: It is tilted at an angle of about 44 degrees to the ecliptic. In about 800 years, Eris will be closer to the Sun than Pluto for some time (see the graph at the right).

The distances of Eris and Pluto from the Sun in the next 1,000 years

As of February 2016[update], Eris has an apparent magnitude of 18.7, making it bright enough to be detectable to some amateur telescopes. A 200-millimeter (7.9 in) telescope with a CCD can detect Eris under favorable conditions.[i] The reason it had not been noticed until now is its steep orbital inclination; searches for large outer Solar System objects tend to concentrate on the ecliptic plane, where most bodies are found.

Because of the high inclination of its orbit, Eris only passes through a few constellations of the traditional Zodiac; it is now in the constellation Cetus. It was in Sculptor from 1876 until 1929 and Phoenix from roughly 1840 until 1875. In 2036 it will enter Pisces and stay there until 2065, when it will enter Aries.[47] It will then move into the northern sky, entering Perseus in 2128 and Camelopardalis (where it will reach its northernmost declination) in 2173.

In 2005, the diameter of Eris was measured to be 7006239700000000000♠2397±100 km, using images from the Hubble Space Telescope (HST).[50][52] The size of an object is determined from its absolute magnitude (H) and the albedo (the amount of light it reflects). At a distance of 97 AU, an object with a diameter of 3,000 km would have an angular size of 40 milliarcseconds,[15] which is directly measurable with the Hubble Space Telescope. Although resolving such small objects is at the very limit of its capabilities,[j] sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.[k]

This makes Eris around the same size as Pluto, which is 7006237200000000000♠2372±4 km across. It also indicates an albedo of 0.96, higher than that of any other large body in the Solar System except Enceladus.[8] It is speculated that the high albedo is due to the surface ices being replenished because of temperature fluctuations as Eris's eccentric orbit takes it closer and farther from the Sun.[54]

In 2007, a series of observations of the largest trans-Neptunian objects with the Spitzer Space Telescope gave an estimate of Eris's diameter of 7006260000000000000♠2600+400
−200 km.[51] The Spitzer and Hubble estimates overlap in the range of 2,400–2,500 km, 4–8% larger than Pluto. Astronomers now suspect that Eris's spin axis is currently pointing toward the Sun, which would make the sunlit hemisphere warmer than average and skew any infrared measurements toward higher values.[9] So the outcome from the 2010 Chile occultation is actually more in line with the Hubble result from 2005.[9]

In November 2010, Eris was the subject of one of the most distant stellar occultations yet from Earth.[9] Preliminary data from this event cast doubt on previous size estimates.[9] The teams announced their final results from the occultation in October 2011, with an estimated diameter of 7006232600000000000♠2326+6
−6 km.[8] The mass of Eris can be calculated with much greater precision. Based on the currently accepted value for Dysnomia's period—15.774 days—[10][55] Eris is 27 percent more massive than Pluto. If the 2011 occultation results are used, then Eris has a density of 7003252000000000000♠2.52±0.07 g/cm3,[d] substantially denser than Pluto, and thus must be composed largely of rocky materials.[8]

In July 2015, after nearly ten years of Eris being considered the ninth-largest object known to directly orbit the sun, close-up imagery from the New Horizons mission more accurately determined Pluto's volume to be slightly larger than Eris's, rather than slightly smaller as previously thought. Eris is now the tenth-largest object known to directly orbit the sun by volume, though not by mass.[57]

The infrared spectrum of Eris, compared to that of Pluto, shows the marked similarities between the two bodies. Arrows denote methane absorption lines.

The discovery team followed up their initial identification of Eris with spectroscopic observations made at the 8 m Gemini North Telescope in Hawaii on January 25, 2005. Infrared light from the object revealed the presence of methane ice, indicating that the surface may be similar to that of Pluto, which at the time was the only TNO known to have surface methane, and of Neptune's moon Triton, which also has methane on its surface.[58] No surface details can be resolved from Earth or its orbit with any instrument currently available.

Due to Eris's distant eccentric orbit, its surface temperature is estimated to vary between about 30 and 56 K (−243.2 and −217.2 °C).[2]

Unlike the somewhat reddish Pluto and Triton, Eris appears almost white.[2] Pluto's reddish color is thought to be due to deposits of tholins on its surface, and where these deposits darken the surface, the lower albedo leads to higher temperatures and the evaporation of methane deposits. In contrast, Eris is far enough from the Sun that methane can condense onto its surface even where the albedo is low. The condensation of methane uniformly over the surface reduces any albedo contrasts and would cover up any deposits of red tholins.[59]

Even though Eris can be up to three times farther from the Sun than Pluto, it approaches close enough that some of the ices on the surface might warm enough to sublime. Because methane is highly volatile, its presence shows either that Eris has always resided in the distant reaches of the Solar System, where it is cold enough for methane ice to persist, or that the celestial body has an internal source of methane to replenish gas that escapes from its atmosphere. This contrasts with observations of another discovered TNO, Haumea, which reveal the presence of water ice but not methane.[60]

In 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris), using the newly commissioned laser guide star adaptive optics system.[61] Images taken on September 10 revealed a moon in orbit around Eris. In keeping with the "Xena" nickname already in use for Eris, Brown's team nicknamed the moon "Gabrielle", after the television warrior princess' sidekick. When Eris received its official name from the IAU, the moon received the name Dysnomia, after the Greek goddess of lawlessness who was Eris's daughter. Brown says he picked it for similarity to his wife's name, Diane. The name also retains an oblique reference to Eris's old informal name Xena, portrayed on TV by Lucy Lawless.[62]

It was calculated that a flyby mission to Eris could take 24.66 years using a Jupiter gravity assist, based on launch dates of 3 April 2032 or 7 April 2044. Eris would be 92.03 or 90.19 AU from the Sun when the spacecraft arrives.[63]

^As of February 2016[update], Sedna is 85.7 astronomical units (1.282×1010 km; 7.97×109 mi) from the Sun,[19] whereas Eris is 96.3 astronomical units (1.441×1010 km; 8.95×109 mi) from the Sun.[13] Eris is still close to its 1977 aphelion (furthest distance from the Sun), whereas Sedna is nearing its 2076 perihelion (closest approach to the Sun).[20] Sedna will overtake Eris as the farthest presently known spherical minor planet in 2114.[20]

^Brown is joking on this point. It was in fact the Disney character Pluto that was named after the newly discovered "planet", though Venetia Phair, Pluto's christener, had to counter accusations her whole life that she named the planet after a cartoon dog.[31]

^The Resolution of the High Resolution Channel of the ACS is 40 marcsec (milliarcseconds) and the size of 1 pixel is ~25 marcsec i.e. ~1875 km at the distance of Eris.

^The reference to 'direct' measurement by the HST should not mislead into thinking that this method is as 'direct' and model-independent as measuring say Neptune's size. Basically, the method consists in finding the statistically best fit to a smeared image of the size of less than 2 pixels by comparing it with smeared images of the background stars, using a given computer model of the optics (PSF). A non-technical description of the method is given on Brown's page, a detailed description of this approach and its limitations are discussed in a paper on Quaoar[53]

1.
433 Eros
–
433 Eros is an S-type near-Earth asteroid approximately 34. 4×11. 2×11.2 kilometres in size, the second-largest near-Earth asteroid after 1036 Ganymed. It was discovered in 1898 and was the first near-Earth asteroid discovered and it was the first asteroid orbited by an Earth probe. It belongs to the Amor group, Eros is a Mars-crosser asteroid, the first known to come within the orbit of Mars. Objects in such an orbit can remain there for only a few hundred years before the orbit is perturbed by gravitational interactions. It is a potential Earth impactor, about five times larger than the impactor that created Chicxulub crater, the NEAR Shoemaker probe visited Eros twice, first with a 1998 flyby, and then by orbiting it in 2000 when it extensively photographed its surface. On February 12,2001, at the end of its mission, Eros was discovered on 13 August 1898 by Gustav Witt in Berlin and Auguste Charlois at Nice. Witt was taking a 2-hour exposure of Beta Aquarii to secure astrometric positions of asteroid 185 Eunike, a similar program was then carried out, during a closer approach, in 1930–1931 by Harold Spencer Jones. The value obtained by this program was considered definitive until 1968, Eros was the first asteroid detected by the Arecibo Observatorys radar system. Eros was one of the first asteroids visited by a spacecraft, the first one orbited, NASA spacecraft NEAR Shoemaker entered orbit around Eros in 2000, and landed in 2001. Eros is named after the Greek god of love, Erōs and it is pronounced /ˈɪərɒs/ EER-os or sometimes /ˈɛrɒs/ ERR-os. The rarely used form of the name is Erotian /ᵻˈroʊʃən/. Surface gravity depends on the distance from a spot on the surface to the center of a bodys mass, eross surface gravity varies greatly because Eros is not a sphere but an elongated peanut-shaped object. The daytime temperature on Eros can reach about 100 °C at perihelion, nighttime measurements fall near −150 °C. Eross density is 2.67 g/cm3, about the same as the density of Earths crust and it rotates once every 5.27 hours. NEAR scientists have found that most of the larger rocks strewn across Eros were ejected from a crater in an impact approximately 1 billion years ago. This event may also be responsible for the 40 percent of the Erotian surface that is devoid of craters smaller than 0.5 kilometers across and it was originally thought that the debris thrown up by the collision filled in the smaller craters. An analysis of crater densities over the surface indicates that the areas with lower density are within 9 kilometers of the impact point. Some of the lower density areas were found on the side of the asteroid

2.
Dysnomia (moon)
–
Dysnomia —officially Eris I Dysnomia—is the only known moon of the dwarf planet Eris. Observations taken on 10 September revealed a moon in orbit around Eris, in keeping with the Xena nickname that was already in use for Eris, the moon was nicknamed Gabrielle by its discoverers, after Xenas sidekick. It is 60 times fainter than Eris in the K band and 480 times fainter in the V band, which means a very different, and quite redder, spectrum, indicating a significantly darker surface. Assuming its albedo is five times lower than Eriss, its diameter would be 685±50 km, of the Solar Systems dwarf planet and minor planet moons, only Charon is definitely larger than Dysnomia. Combining Keck and Hubble observations, the satellite was used to determine the mass of Eris and its orbital period is calculated to be 15. 774±0.002 d. These observations indicate that Dysnomia has an orbit around Eris. This shows that the mass of Eris is 1.27 times that of Pluto, astronomers now know that the four brightest Kuiper belt objects have satellites. Among the fainter members of the only about 10% are known to have satellites. This is thought to imply that collisions between large KBOs have been frequent in the past, impacts between bodies of the order of 1000 km across would throw off large amounts of material that would coalesce into a moon. A similar mechanism is thought to have led to the formation of the Moon when Earth was struck by a giant impactor early in the history of the Solar System. Mike Brown, the discoverer, chose the name Dysnomia due to a number of associations it had for him. Dysnomia, the daughter of Eris, fits the general historically established pattern of naming moons after lesser gods associated with the primary god, also, the English translation of Dysnomia, lawlessness, echoes Lucy Lawless, the actress famous for starring in Xena, Warrior Princess on television. Before receiving their names, Eris and Dysnomia were nicknamed Xena and Gabrielle. James Christy, who discovered Charon, followed the principle established with Pluto by choosing a name which shared its first four letters with his wifes name, Charlene. Dysnomia, similarly, has the same first letter as Browns wife, Diane, and Brown uses the nickname Dy /ˈdaɪ/ for the moon, because of this, Brown pronounces the full name /daɪsˈnoʊmiə/, with a long y. Media related to Dysnomia at Wikimedia Commons

3.
Hubble Space Telescope
–
The Hubble Space Telescope is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile, with a 2. 4-meter mirror, Hubbles four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubbles orbit outside the distortion of Earths atmosphere allows it to take extremely high-resolution images, Hubble has recorded some of the most detailed visible light images ever, allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe. The HST was built by the United States space agency NASA, the Space Telescope Science Institute selects Hubbles targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft. Space telescopes were proposed as early as 1923, Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster. When finally launched in 1990, Hubbles main mirror was found to have been ground incorrectly, the optics were corrected to their intended quality by a servicing mission in 1993. Hubble is the telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired, upgraded, the fifth mission was canceled on safety grounds following the Columbia disaster. However, after spirited public discussion, NASA administrator Mike Griffin approved the fifth servicing mission, the telescope is operating as of 2017, and could last until 2030–2040. Its scientific successor, the James Webb Space Telescope, is scheduled for launch in 2018, the history of the Hubble Space Telescope can be traced back as far as 1946, to the astronomer Lyman Spitzers paper Astronomical advantages of an extraterrestrial observatory. In it, he discussed the two advantages that a space-based observatory would have over ground-based telescopes. First, the resolution would be limited only by diffraction, rather than by the turbulence in the atmosphere. Second, a telescope could observe infrared and ultraviolet light. Spitzer devoted much of his career to pushing for the development of a space telescope, space-based astronomy had begun on a very small scale following World War II, as scientists made use of developments that had taken place in rocket technology. An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, oAO-1s battery failed after three days, terminating the mission. It was followed by OAO-2, which carried out observations of stars and galaxies from its launch in 1968 until 1972. The continuing success of the OAO program encouraged increasingly strong consensus within the community that the LST should be a major goal

4.
Michael E. Brown
–
Michael E. Brown is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology since 2003. His team has discovered many objects, notably the dwarf planet Eris. He is the author of How I Killed Pluto and Why It Had It Coming and he earned his A. B. in physics from Princeton University in 1987, where he was a member of the Princeton Tower Club. He did his studies at the University of California, Berkeley where he earned an M. A. degree in astronomy in 1990. Brown is well known in the community for his surveys for distant objects orbiting the Sun. His team has discovered many trans-Neptunian objects, Browns team famously named Eris and its moon Dysnomia with the informal names Xena and Gabrielle, respectively, after the two main characters of Xena, Warrior Princess. Brown originally indicated his support for Ortizs team being given credit for the discovery of Haumea, however, the Minor Planet Center only needs precise enough orbit determination on the object in order to provide discovery credit, which Ortiz provided. The then director of the IAA, José Carlos del Toro, distanced himself from Ortiz, Brown petitioned the International Astronomical Union to credit his team rather than Ortiz as the discoverers of Haumea. The IAU has deliberately not acknowledged a discoverer of Haumea, the discovery date and location are listed as March 7,2003 at Ortizs Sierra Nevada Observatory. However, the IAU accepted Browns suggested name of Haumea, which fit the names of Haumeas two moons, rather than Ortizs Ataecina. In January 2016, Brown and fellow Caltech astronomer, Konstantin Batygin, proposed the existence of Planet Nine, the two astronomers gave a recorded interview in which they described their method and reasoning for proposing Planet 9 on January 20,2016. In 2010 Brown published a memoir of his discoveries and surrounding family life, How I Killed Pluto, Brown was named one of Times 100 most influential people of 2006. In 2007 he received Caltechs annual Feynman Prize, Caltechs most prestigious teaching honor, asteroid 11714 Mikebrown, discovered on 28 April 1998, was named in his honor. In 2012, Brown was awarded the Kavli Prize in Astrophysics, Brown married Diane Binney on March 1,2003. They have one daughter, Lilah Binney Brown and he is likes being known as the Pluto Killer so uses the Twitter handle plutokiller. Konstantin Batygin Planet Nine Notes References Wilkinson, Alex

5.
Chad Trujillo
–
Chadwick A. Chad Trujillo is an American astronomer, discoverer of minor planets and the co-discoverer of Eris, the most massive dwarf planet known in the Solar System. Trujillo works with software and has examined the orbits of the numerous trans-Neptunian objects. In late August 2005, it was announced that Trujillo, along with Michael E. Brown, as a result of the discovery of the satellite Dysnomia, Eris was the first TNO known to be more massive than Pluto. Trujillo attended Oak Park and River Forest High School in Oak Park, Trujillo was later a postdoctoral scholar at Caltech, and is currently an astronomer at the Gemini Observatory in Hawaii. He studies the Kuiper belt and the outer Solar System, the main-belt asteroid 12101 Trujillo is named for him. Trujillo is credited by the Minor Planet Center with the discovery and co-discovery of 50 numbered minor planets between 1996 and 2007, including many trans-Neptunian objects from the Kuiper belt, makemake, co-discovered with Brown and Rabinowitz in 2005, one of the first 5 official dwarf planets

6.
Eris (mythology)
–
Eris is the Greek goddess of strife and discord. Her name is the equivalent of Latin Discordia, which means discord, Eris Greek opposite is Harmonia, whose Latin counterpart is Concordia. Homer equated her with the war-goddess Enyo, whose Roman counterpart is Bellona, the dwarf planet Eris is named after the goddess, as is the religion Discordianism. She then hurled down bitterness equally between both sides as she walked through the onslaught making mens pain heavier and she also has a son whom she named Strife. Enyo is mentioned in Book 5, and Zeus sends Strife to rouse the Achaeans in Book 11, the most famous tale of Eris recounts her initiating the Trojan War by causing the Judgement of Paris. The hapless Paris, Prince of Troy, was appointed to select the fairest by Zeus, the goddesses stripped naked to try to win Paris decision, and also attempted to bribe him. Hera offered political power, Athena promised infinite wisdom, and Aphrodite tempted him with the most beautiful woman in the world, Helen, wife of Menelaus of Sparta. While Greek culture placed an emphasis on prowess and power, Paris chose to award the apple to Aphrodite, thereby dooming his city. In Nonnus Dionysiaca,2.356, when Typhon prepares to battle with Zeus, Eris was Typhons escort in the melée, another story of Eris includes Hera, and the love of Polytekhnos and Aedon. They claimed to each other more than Hera and Zeus were in love. This angered Hera, so she sent Eris to rack discord upon them, Polytekhnos was finishing off a chariot board, and Aedon a web she had been weaving. Eris said to them, Whosoever finishes thine task last shall have to present the other with a female servant, but Polytekhnos was not happy by his defeat, so he came to Khelidon, Aedons sister, and raped her. He then disguised her as a slave, presenting her to Aedon, when Aedon discovered this was indeed her sister, she chopped up Polytekhnos son and fed him to Polytekhnos. The gods were not pleased, so turned them all into birds. The Discordian version of Eris is considerably lighter in comparison to the rather malevolent Graeco-Roman original and she told him that She had always liked the Old Greeks, but that they cannot be trusted with historic matters. They were, She added, victims of indigestion, you know, the story of Eris being snubbed and indirectly starting the Trojan War is recorded in the Principia, and is referred to as the Original Snub. The Principia Discordia states that her parents may be as described in Greek legend and she is the Goddess of Disorder and Being, whereas her sister Aneris is the goddess of Order and Non-Being. The concept of Eris as developed by the Principia Discordia is used and expanded upon in the fiction work The Illuminatus

7.
Minor planet
–
A minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor exclusively classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, as of 2016, the orbits of 709,706 minor planets were archived at the Minor Planet Center,469,275 of which had received permanent numbers. The first minor planet to be discovered was Ceres in 1801, the term minor planet has been used since the 19th century to describe these objects. The term planetoid has also used, especially for larger objects such as those the International Astronomical Union has called dwarf planets since 2006. Historically, the asteroid, minor planet, and planetoid have been more or less synonymous. This terminology has become complicated by the discovery of numerous minor planets beyond the orbit of Jupiter. A Minor planet seen releasing gas may be classified as a comet. Before 2006, the IAU had officially used the term minor planet, during its 2006 meeting, the IAU reclassified minor planets and comets into dwarf planets and small Solar System bodies. Objects are called dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium, all other minor planets and comets are called small Solar System bodies. The IAU stated that the minor planet may still be used. However, for purposes of numbering and naming, the distinction between minor planet and comet is still used. Hundreds of thousands of planets have been discovered within the Solar System. The Minor Planet Center has documented over 167 million observations and 729,626 minor planets, of these,20,570 have official names. As of March 2017, the lowest-numbered unnamed minor planet is 1974 FV1, as of March 2017, the highest-numbered named minor planet is 458063 Gustavomuler. There are various broad minor-planet populations, Asteroids, traditionally, most have been bodies in the inner Solar System. Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on distance, is used, Apohele asteroids orbit inside of Earths perihelion distance. Aten asteroids, those that have semi-major axes of less than Earths, Apollo asteroids are those asteroids with a semimajor axis greater than Earths, while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers, amor asteroids are those near-Earth asteroids that approach the orbit of Earth from beyond, but do not cross it

8.
Dwarf planet
–
A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. The International Astronomical Union currently recognizes five dwarf planets, Ceres, Pluto, Haumea, Makemake, another hundred or so known objects in the Solar System are suspected to be dwarf planets. Individual astronomers recognize several of these, and in August 2011 Mike Brown published a list of 390 candidate objects, Stern states that there are more than a dozen known dwarf planets. Only two of these bodies, Ceres and Pluto, have observed in enough detail to demonstrate that they actually fit the IAUs definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto and they subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 are to be named under the assumption that they are dwarf planets. The classification of bodies in other systems with the characteristics of dwarf planets has not been addressed. Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for some decades considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the asteroid for the smaller bodies. With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets and it was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was more than ten times as massive as the largest object in the asteroid belt, Ceres. In the 1990s, astronomers began to find objects in the region of space as Pluto. Many of these shared several of Plutos key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects and this led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the now known as dwarf planets. By 2005, three trans-Neptunian objects comparable in size to Pluto had been reported and it became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified. Astronomers were also confident that more objects as large as Pluto would be discovered, Eris was discovered in January 2005, it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet. As a consequence, the became a matter of intense debate during the IAU General Assembly in August 2006. The IAUs initial draft proposal included Charon, Eris, and Ceres in the list of planets, dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits. The IAUs final Resolution 5A preserved this three-category system for the bodies orbiting the Sun

9.
Trans-Neptunian object
–
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune,30 astronomical units. Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, the first trans-Neptunian object to be discovered was Pluto in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly,1992 QB1, as of February 2017 over 2,300 trans-Neptunian objects appear on the Minor Planet Centers List of Transneptunian Objects. Of these TNOs,2,000 have a perihelion farther out than Neptune, as of November 2016,242 of these have their orbits well-enough determined that they have been given a permanent minor planet designation. The largest known object is Pluto, followed by Eris,2007 OR10, Makemake. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space, though treatments vary and a few objects such as Sedna do not fit easily into any division. The orbit of each of the planets is slightly affected by the influences of the other planets. Discrepancies in the early 1900s between the observed and expected orbits of Uranus and Neptune suggested that there were one or more additional planets beyond Neptune, the search for these led to the discovery of Pluto in February 1930, which was too small to explain the discrepancies. Revised estimates of Neptunes mass from the Voyager 2 flyby in 1989 showed that the problem was spurious, Pluto was easiest to find because it has the highest apparent magnitude of all known trans-Neptunian objects. It also has an inclination to the ecliptic than most other large TNOs. After Plutos discovery, American astronomer Clyde Tombaugh continued searching for years for similar objects. For a long time, no one searched for other TNOs as it was believed that Pluto. Only after the 1992 discovery of a second TNO,1992 QB1, a broad strip of the sky around the ecliptic was photographed and digitally evaluated for slowly moving objects. Hundreds of TNOs were found, with diameters in the range of 50 to 2,500 kilometers, Pluto and Eris were eventually classified as dwarf planets by the International Astronomical Union. Kuiper belt objects are classified into the following two groups, Resonant objects are locked in an orbital resonance with Neptune. Objects with a 1,2 resonance are called twotinos, and objects with a 2,3 resonance are called plutinos, after their most prominent member, classical Kuiper belt objects have no such resonance, moving on almost circular orbits, unperturbed by Neptune. Examples are 1992 QB1,50000 Quaoar and Makemake, the scattered disc contains objects farther from the Sun, usually with very irregular orbits. A typical example is the most massive known TNO, Eris, scattered-extended —Scattered-extended objects have a Tisserand parameter greater than 3 and have a time-averaged eccentricity greater than 0

10.
Plutoid
–
A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i. e. a body orbiting beyond Neptune that is massive enough to be rounded in shape. The term plutoid was adopted by the International Astronomical Union working group Committee on Small Bodies Nomenclature, the term plutoid is not widely used by astronomers, though ice dwarf is not uncommon. There are thought to be thousands of plutoids in the Solar System, the IAU developed this category of astronomical objects as a consequence of its 2006 resolution defining the word planet. Satellites of plutoids are not plutoids themselves, on 24 August 2006, the IAU decided to reclassify Pluto as a dwarf planet, requiring that a planet must clear the neighbourhood around its orbit. The General Assembly of the IAU further resolved, Pluto is recognized as the prototype of a new category of Trans-Neptunian Objects and this new category had been proposed under the name pluton or a plutonian object earlier in the General Assembly. The former was rejected, in part because pluton is a geological term, the definition of the category also fluctuated during its early stages. Ultimately, the final resolution left the definition, like the name. The term was announced after the Executive Committee meeting, along with a simplified definition. However, due to an error, the IAU Working Group for Planetary System Nomenclature was not consulted. As of 2009, the term had not been accepted by the scientific community. Prior to the emergence of the term plutoid as an IAU-sanctioned categorization, the term ice dwarf saw some adoption as a near-synonym to the eventual term plutoid. There are large numbers of objects in the Oort cloud. However, it is not clear whether all so-categorized ice dwarfs are actually icier than icy asteroids such as Ceres, nonetheless, Ceres is sometimes called a terrestrial dwarf to distinguish it from Pluto and Eris. This conception foreshadowed the reclassification of Pluto to dwarf planet and plutoid after the discovery of Eris, with the creation of the term dwarf planet, some ambiguity was created as to which of two IAU bodies would be responsible for naming dwarf planets. Eris had been named through the IAU Committee on Small Body Nomenclature, along with announcing the name plutoid, the IAU decision of 11 June 2008 institutionalized this cooperative process involving the two bodies in the naming of new plutoids. In keeping with minor planet naming guidelines, priority will be given to names proposed by the discovery teams, and plutoids may not share a name with a small Solar System body. When the definition of planet was instated at the IAU General Assembly of 2006, Ceres, Pluto. However, precise regulations as to how hydrostatic equilibrium would be measured were left undefined for the time being, without an official procedure for calculating the lower bound of size to be a dwarf planet, no further bodies could be formally recognized as either dwarf planets or plutoids

11.
Scattered disc
–
The scattered disc is a distant circumstellar disc in the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40° and these extreme orbits are thought to be the result of gravitational scattering by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune. Although the closest scattered-disc objects approach the Sun at about 30–35 AU and this makes scattered objects among the most distant and coldest objects in the Solar System. Eventually, perturbations from the giant planets send such objects towards the Sun, many Oort cloud objects are also thought to have originated in the scattered disc. Detached objects are not sharply distinct from scattered disc objects, during the 1980s, the use of CCD-based cameras in telescopes made it possible to directly produce electronic images that could then be readily digitized and transferred to digital images. Because the CCD captured more light than film and the blinking could now be done at a computer screen. A flood of new discoveries was the result, over a thousand objects were detected between 1992 and 2006. The first scattered-disc object to be recognised as such was 1996 TL66, three more were identified by the same survey in 1999,1999 CV118,1999 CY118, and 1999 CF119. The first object presently classified as an SDO to be discovered was 1995 TL8, as of 2011, over 200 SDOs have been identified, including 2007 UK126,2002 TC302, Eris, Sedna and 2004 VN112. Known trans-Neptunian objects are divided into two subpopulations, the Kuiper belt and the scattered disc. A third reservoir of trans-Neptunian objects, the Oort cloud, has been hypothesized, some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with detached objects. Those in 3,2 resonances are known as plutinos, because Pluto is the largest member of their group, in contrast to the Kuiper belt, the scattered-disc population can be disturbed by Neptune. Scattered-disc objects come within range of Neptune at their closest approaches. Some objects, like 1999 TD10, blur the distinction and the Minor Planet Center, the MPC also makes a clear distinction between the Kuiper belt and the scattered disc, separating those objects in stable orbits from those in scattered orbits. Another term used is scattered Kuiper-belt object for bodies of the scattered disc and this delineation is inadequate over the age of the Solar System, since bodies trapped in resonances could pass from a scattering phase to a non-scattering phase numerous times. That is, trans-Neptunian objects could travel back and forth between the Kuiper belt and the disc over time. In the a >30 AU region, the region of the Solar System populated by objects with semi-major axes greater than 30 AU, the Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered-disc object. Under this definition, an object with a greater than 40 AU could be classified as outside the scattered disc

12.
Minor planet moon
–
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, the first modern era mention of the possibility of an asteroid satellite was in connection with an occultation of the bright star Gamma Ceti by the minor planet Hebe in 1977. The observer, amateur astronomer Paul D. Maley, detected an unmistakable 0.5 second disappearance of this naked eye star from a site near Victoria, many hours later, several observations were reported in Mexico attributed to the occultation by Hebe itself. Although not confirmed this documents the first formally documented case of a companion of an asteroid. As of October 2016, there are over 300 minor planets known to have moons, in addition to the terms satellite and moon, the term binary is sometimes used for minor planets with moons, and triple for minor planets with two moons. If one object is much bigger it can be referred to as the primary, when binary minor planets are similar in size, the Minor Planet Center refers to them as binary companions instead of referring to the smaller body as a satellite. A good example of a true binary is the 90 Antiope system, small satellites are often referred to as moonlets. As of February 2017, over 330 moons of planets have been discovered. For example, in 1978, stellar occultation observations were claimed as evidence of a satellite for the asteroid 532 Herculina, however, later more-detailed imaging by the Hubble Telescope did not reveal a satellite, and the current consensus is that Herculina does not have a significant satellite. There were other reports of asteroids having companions in the following years. In 1993, the first asteroid moon was confirmed when the Galileo probe discovered the small Dactyl orbiting 243 Ida in the asteroid belt, the second was discovered around 45 Eugenia in 1998. In 2001,617 Patroclus and its same-sized companion Menoetius became the first known asteroids in the Jupiter trojans. The first trans-Neptunian binary after Pluto–Charon,1998 WW31, was resolved in 2002. Triple asteroids, or trinary asteroids, are known since 2005 and this was followed by the discovery of a second moon orbiting 45 Eugenia. Also in 2005, the Kuiper belt object Haumea was discovered to have two moons, making it the second KBO after Pluto known to have more than one moon, additionally,216 Kleopatra and 93 Minerva were discovered to be trinary asteroids in 2008 and 2009 respectively. Since the first few trinary asteroids were discovered, more continue to be discovered at a rate of one a year. Most recently discovered was a moon orbiting the belt asteroid 130 Elektra. List of multiple planets, The data about the populations of binary objects are still patchy

13.
Perihelion and aphelion
–
The perihelion is the point in the orbit of a celestial body where it is nearest to its orbital focus, generally a star. It is the opposite of aphelion, which is the point in the orbit where the body is farthest from its focus. The word perihelion stems from the Ancient Greek words peri, meaning around or surrounding, aphelion derives from the preposition apo, meaning away, off, apart. According to Keplers first law of motion, all planets, comets. Hence, a body has a closest and a farthest point from its parent object, that is, a perihelion. Each extreme is known as an apsis, orbital eccentricity measures the flatness of the orbit. Because of the distance at aphelion, only 93. 55% of the solar radiation from the Sun falls on a given area of land as does at perihelion. However, this fluctuation does not account for the seasons, as it is summer in the northern hemisphere when it is winter in the southern hemisphere and vice versa. Instead, seasons result from the tilt of Earths axis, which is 23.4 degrees away from perpendicular to the plane of Earths orbit around the sun. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, in the northern hemisphere, summer occurs at the same time as aphelion. Despite this, there are larger land masses in the northern hemisphere, consequently, summers are 2.3 °C warmer in the northern hemisphere than in the southern hemisphere under similar conditions. Apsis Ellipse Solstice Dates and times of Earths perihelion and aphelion, 2000–2025 from the United States Naval Observatory

14.
Astronomical unit
–
The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a component in the definition of another unit of astronomical length. A variety of symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union used the symbol A for the astronomical unit, in 2006, the International Bureau of Weights and Measures recommended ua as the symbol for the unit. In 2012, the IAU, noting that various symbols are presently in use for the astronomical unit, in the 2014 revision of the SI Brochure, the BIPM used the unit symbol au. In ISO 80000-3, the symbol of the unit is ua. Earths orbit around the Sun is an ellipse, the semi-major axis of this ellipse is defined to be half of the straight line segment that joins the aphelion and perihelion. The centre of the sun lies on this line segment. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, knowing Earths shift and a stars shift enabled the stars distance to be calculated. But all measurements are subject to some degree of error or uncertainty, improvements in precision have always been a key to improving astronomical understanding. Improving measurements were continually checked and cross-checked by means of our understanding of the laws of celestial mechanics, the expected positions and distances of objects at an established time are calculated from these laws, and assembled into a collection of data called an ephemeris. NASAs Jet Propulsion Laboratory provides one of several ephemeris computation services, in 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Equivalently, by definition, one AU is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass. As with all measurements, these rely on measuring the time taken for photons to be reflected from an object. However, for precision the calculations require adjustment for such as the motions of the probe. In addition, the measurement of the time itself must be translated to a scale that accounts for relativistic time dilation

15.
Orbital eccentricity
–
The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is an orbit, values between 0 and 1 form an elliptical orbit,1 is a parabolic escape orbit. The term derives its name from the parameters of conic sections and it is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the galaxy. In a two-body problem with inverse-square-law force, every orbit is a Kepler orbit, the eccentricity of this Kepler orbit is a non-negative number that defines its shape. The limit case between an ellipse and a hyperbola, when e equals 1, is parabola, radial trajectories are classified as elliptic, parabolic, or hyperbolic based on the energy of the orbit, not the eccentricity. Radial orbits have zero angular momentum and hence eccentricity equal to one, keeping the energy constant and reducing the angular momentum, elliptic, parabolic, and hyperbolic orbits each tend to the corresponding type of radial trajectory while e tends to 1. For a repulsive force only the trajectory, including the radial version, is applicable. For elliptical orbits, a simple proof shows that arcsin yields the projection angle of a circle to an ellipse of eccentricity e. For example, to view the eccentricity of the planet Mercury, next, tilt any circular object by that angle and the apparent ellipse projected to your eye will be of that same eccentricity. From Medieval Latin eccentricus, derived from Greek ἔκκεντρος ekkentros out of the center, from ἐκ- ek-, eccentric first appeared in English in 1551, with the definition a circle in which the earth, sun. Five years later, in 1556, a form of the word was added. The eccentricity of an orbit can be calculated from the state vectors as the magnitude of the eccentricity vector, e = | e | where. For elliptical orbits it can also be calculated from the periapsis and apoapsis since rp = a and ra = a, where a is the semimajor axis. E = r a − r p r a + r p =1 −2 r a r p +1 where, rp is the radius at periapsis. For Earths annual orbit path, ra/rp ratio = longest_radius / shortest_radius ≈1.034 relative to center point of path, the eccentricity of the Earths orbit is currently about 0.0167, the Earths orbit is nearly circular. Venus and Neptune have even lower eccentricity, over hundreds of thousands of years, the eccentricity of the Earths orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets. The table lists the values for all planets and dwarf planets, Mercury has the greatest orbital eccentricity of any planet in the Solar System. Such eccentricity is sufficient for Mercury to receive twice as much solar irradiation at perihelion compared to aphelion, before its demotion from planet status in 2006, Pluto was considered to be the planet with the most eccentric orbit

16.
Mean anomaly
–
In celestial mechanics, the mean anomaly is an angle used in calculating the position of a body in an elliptical orbit in the classical two-body problem. Define T as the time required for a body to complete one orbit. In time T, the radius vector sweeps out 2π radians or 360°. The average rate of sweep, n, is then n =2 π T or n =360 ∘ T, define τ as the time at which the body is at the pericenter. From the above definitions, a new quantity, M, the mean anomaly can be defined M = n, because the rate of increase, n, is a constant average, the mean anomaly increases uniformly from 0 to 2π radians or 0° to 360° during each orbit. It is equal to 0 when the body is at the pericenter, π radians at the apocenter, if the mean anomaly is known at any given instant, it can be calculated at any later instant by simply adding n δt where δt represents the time difference. Mean anomaly does not measure an angle between any physical objects and it is simply a convenient uniform measure of how far around its orbit a body has progressed since pericenter. The mean anomaly is one of three parameters that define a position along an orbit, the other two being the eccentric anomaly and the true anomaly. Define l as the longitude, the angular distance of the body from the same reference direction. Thus mean anomaly is also M = l − ϖ, mean angular motion can also be expressed, n = μ a 3, where μ is a gravitational parameter which varies with the masses of the objects, and a is the semi-major axis of the orbit. Mean anomaly can then be expanded, M = μ a 3, and here mean anomaly represents uniform angular motion on a circle of radius a

17.
Degree (angle)
–
A degree, usually denoted by °, is a measurement of a plane angle, defined so that a full rotation is 360 degrees. It is not an SI unit, as the SI unit of measure is the radian. Because a full rotation equals 2π radians, one degree is equivalent to π/180 radians, the original motivation for choosing the degree as a unit of rotations and angles is unknown. One theory states that it is related to the fact that 360 is approximately the number of days in a year. Ancient astronomers noticed that the sun, which follows through the path over the course of the year. Some ancient calendars, such as the Persian calendar, used 360 days for a year, the use of a calendar with 360 days may be related to the use of sexagesimal numbers. The earliest trigonometry, used by the Babylonian astronomers and their Greek successors, was based on chords of a circle, a chord of length equal to the radius made a natural base quantity. One sixtieth of this, using their standard sexagesimal divisions, was a degree, Aristarchus of Samos and Hipparchus seem to have been among the first Greek scientists to exploit Babylonian astronomical knowledge and techniques systematically. Timocharis, Aristarchus, Aristillus, Archimedes, and Hipparchus were the first Greeks known to divide the circle in 360 degrees of 60 arc minutes, eratosthenes used a simpler sexagesimal system dividing a circle into 60 parts. Furthermore, it is divisible by every number from 1 to 10 except 7 and this property has many useful applications, such as dividing the world into 24 time zones, each of which is nominally 15° of longitude, to correlate with the established 24-hour day convention. Finally, it may be the case more than one of these factors has come into play. For many practical purposes, a degree is a small enough angle that whole degrees provide sufficient precision. When this is not the case, as in astronomy or for geographic coordinates, degree measurements may be written using decimal degrees, with the symbol behind the decimals. Alternatively, the sexagesimal unit subdivisions can be used. One degree is divided into 60 minutes, and one minute into 60 seconds, use of degrees-minutes-seconds is also called DMS notation. These subdivisions, also called the arcminute and arcsecond, are represented by a single and double prime. For example,40. 1875° = 40° 11′ 15″, or, using quotation mark characters, additional precision can be provided using decimals for the arcseconds component. The older system of thirds, fourths, etc. which continues the sexagesimal unit subdivision, was used by al-Kashi and other ancient astronomers, but is rarely used today

18.
Orbital inclination
–
Orbital inclination measures the tilt of an objects orbit around a celestial body. It is expressed as the angle between a plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth directly above the equator, the plane of the orbit is the same as the Earths equatorial plane. The general case is that the orbit is tilted, it spends half an orbit over the northern hemisphere. If the orbit swung between 20° north latitude and 20° south latitude, then its orbital inclination would be 20°, the inclination is one of the six orbital elements describing the shape and orientation of a celestial orbit. It is the angle between the plane and the plane of reference, normally stated in degrees. For a satellite orbiting a planet, the plane of reference is usually the plane containing the planets equator, for planets in the Solar System, the plane of reference is usually the ecliptic, the plane in which the Earth orbits the Sun. This reference plane is most practical for Earth-based observers, therefore, Earths inclination is, by definition, zero. Inclination could instead be measured with respect to another plane, such as the Suns equator or the invariable plane, the inclination of orbits of natural or artificial satellites is measured relative to the equatorial plane of the body they orbit, if they orbit sufficiently closely. The equatorial plane is the perpendicular to the axis of rotation of the central body. An inclination of 30° could also be described using an angle of 150°, the convention is that the normal orbit is prograde, an orbit in the same direction as the planet rotates. Inclinations greater than 90° describe retrograde orbits, thus, An inclination of 0° means the orbiting body has a prograde orbit in the planets equatorial plane. An inclination greater than 0° and less than 90° also describe prograde orbits, an inclination of 63. 4° is often called a critical inclination, when describing artificial satellites orbiting the Earth, because they have zero apogee drift. An inclination of exactly 90° is an orbit, in which the spacecraft passes over the north and south poles of the planet. An inclination greater than 90° and less than 180° is a retrograde orbit, an inclination of exactly 180° is a retrograde equatorial orbit. For gas giants, the orbits of moons tend to be aligned with the giant planets equator, the inclination of exoplanets or members of multiple stars is the angle of the plane of the orbit relative to the plane perpendicular to the line-of-sight from Earth to the object. An inclination of 0° is an orbit, meaning the plane of its orbit is parallel to the sky. An inclination of 90° is an orbit, meaning the plane of its orbit is perpendicular to the sky

19.
Longitude of the ascending node
–
The longitude of the ascending node is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a direction, called the origin of longitude, to the direction of the ascending node. The ascending node is the point where the orbit of the passes through the plane of reference. Commonly used reference planes and origins of longitude include, For a geocentric orbit, Earths equatorial plane as the plane. In this case, the longitude is called the right ascension of the ascending node. The angle is measured eastwards from the First Point of Aries to the node, for a heliocentric orbit, the ecliptic as the reference plane, and the First Point of Aries as the origin of longitude. The angle is measured counterclockwise from the First Point of Aries to the node, the angle is measured eastwards from north to the node. pp.40,72,137, chap. In the case of a star known only from visual observations, it is not possible to tell which node is ascending. In this case the orbital parameter which is recorded is the longitude of the node, Ω, here, n=<nx, ny, nz> is a vector pointing towards the ascending node. The reference plane is assumed to be the xy-plane, and the origin of longitude is taken to be the positive x-axis, K is the unit vector, which is the normal vector to the xy reference plane. For non-inclined orbits, Ω is undefined, for computation it is then, by convention, set equal to zero, that is, the ascending node is placed in the reference direction, which is equivalent to letting n point towards the positive x-axis. Kepler orbits Equinox Orbital node perturbation of the plane can cause revolution of the ascending node

20.
Argument of periapsis
–
The argument of periapsis, symbolized as ω, is one of the orbital elements of an orbiting body. Parametrically, ω is the angle from the ascending node to its periapsis. For specific types of orbits, words such as perihelion, perigee, periastron, an argument of periapsis of 0° means that the orbiting body will be at its closest approach to the central body at the same moment that it crosses the plane of reference from South to North. An argument of periapsis of 90° means that the body will reach periapsis at its northmost distance from the plane of reference. Adding the argument of periapsis to the longitude of the ascending node gives the longitude of the periapsis, however, especially in discussions of binary stars and exoplanets, the terms longitude of periapsis or longitude of periastron are often used synonymously with argument of periapsis. In the case of equatorial orbits, the argument is strictly undefined, where, ex and ey are the x- and y-components of the eccentricity vector e. In the case of circular orbits it is assumed that the periapsis is placed at the ascending node. Kepler orbit Orbital mechanics Orbital node

21.
Natural satellite
–
A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 178 known natural satellites, four IAU-listed dwarf planets are also known to have natural satellites, Pluto, Haumea, Makemake, and Eris. As of January 2012, over 200 minor-planet moons have been discovered, the Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km across, Earths Moon is 0.27 times the diameter of Earth, the first known natural satellite was the Moon, but it was considered a planet until Copernicus introduction of heliocentrism in 1543. Until the discovery of the Galilean satellites in 1610, however, galileo chose to refer to his discoveries as Planetæ, but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use of the satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word satelles, meaning guard, attendant, or companion, the term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of moon. In 1957, however, the launching of the artificial object Sputnik created a need for new terminology, to further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earths natural satellite, but not when referring to other natural satellites. A few recent authors define moon as a satellite of a planet or minor planet, there is no established lower limit on what is considered a moon. Small asteroid moons, such as Dactyl, have also been called moonlets, the upper limit is also vague. Two orbiting bodies are described as a double body rather than primary. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon, some authors consider the Pluto–Charon system to be a double planet. In contrast, irregular satellites are thought to be captured asteroids possibly further fragmented by collisions, most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and possibly Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects. The material that would have placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to form by this process. Triton is another exception, although large and in a close, circular orbit, its motion is retrograde, most regular moons in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. The only known exception is Saturns natural satellite Hyperion, which rotates chaotically because of the influence of Titan

22.
Spheroid
–
A spheroid, or ellipsoid of revolution, is a quadric surface obtained by rotating an ellipse about one of its principal axes, in other words, an ellipsoid with two equal semi-diameters. If the ellipse is rotated about its axis, the result is a prolate spheroid. If the ellipse is rotated about its axis, the result is an oblate spheroid. If the generating ellipse is a circle, the result is a sphere, because of the combined effects of gravity and rotation, the Earths shape is not quite a sphere but instead is slightly flattened in the direction of its axis of rotation. For that reason, in cartography the Earth is often approximated by an oblate spheroid instead of a sphere, the current World Geodetic System model uses a spheroid whose radius is 6,378.137 km at the equator and 6,356.752 km at the poles. The semi-major axis a is the radius of the spheroid. There are two cases, c < a, oblate spheroid c > a, prolate spheroid The case of a = c reduces to a sphere. An oblate spheroid with c < a has surface area S o b l a t e =2 π a 2 where e 2 =1 − c 2 a 2. The oblate spheroid is generated by rotation about the z-axis of an ellipse with semi-major axis a and semi-minor axis c, therefore e may be identified as the eccentricity. A prolate spheroid with c > a has surface area S p r o l a t e =2 π a 2 where e 2 =1 − a 2 c 2. The prolate spheroid is generated by rotation about the z-axis of an ellipse with semi-major axis c and semi-minor axis a and these formulas are identical in the sense that the formula for Soblate can be used to calculate the surface area of a prolate spheroid and vice versa. However, e then becomes imaginary and can no longer directly be identified with the eccentricity, both of these results may be cast into many other forms using standard mathematical identities and relations between parameters of the ellipse. The volume inside a spheroid is 4π/3a2c ≈4. 19a2c, if A = 2a is the equatorial diameter, and C = 2c is the polar diameter, the volume is π/6A2C ≈0. 523A2C. Both of these curvatures are always positive, so every point on a spheroid is elliptic. These are just two of different parameters used to define an ellipse and its solid body counterparts. The most common shapes for the density distribution of protons and neutrons in an atomic nucleus are spherical, prolate and oblate spheroidal, deformed nuclear shapes occur as a result of the competition between electromagnetic repulsion between protons, surface tension and quantum shell effects. An extreme example of a planet in science fiction is Mesklin, in Hal Clements novel Mission of Gravity. The prolate spheroid is the shape of the ball in several sports, several moons of the Solar system approximate prolate spheroids in shape, though they are actually triaxial ellipsoids

23.
Volume
–
Volume is the quantity of three-dimensional space enclosed by a closed surface, for example, the space that a substance or shape occupies or contains. Volume is often quantified numerically using the SI derived unit, the cubic metre, three dimensional mathematical shapes are also assigned volumes. Volumes of some simple shapes, such as regular, straight-edged, volumes of a complicated shape can be calculated by integral calculus if a formula exists for the shapes boundary. Where a variance in shape and volume occurs, such as those that exist between different human beings, these can be calculated using techniques such as the Body Volume Index. One-dimensional figures and two-dimensional shapes are assigned zero volume in the three-dimensional space, the volume of a solid can be determined by fluid displacement. Displacement of liquid can also be used to determine the volume of a gas, the combined volume of two substances is usually greater than the volume of one of the substances. However, sometimes one substance dissolves in the other and the volume is not additive. In differential geometry, volume is expressed by means of the volume form, in thermodynamics, volume is a fundamental parameter, and is a conjugate variable to pressure. Any unit of length gives a unit of volume, the volume of a cube whose sides have the given length. For example, a cubic centimetre is the volume of a cube whose sides are one centimetre in length, in the International System of Units, the standard unit of volume is the cubic metre. The metric system also includes the litre as a unit of volume, thus 1 litre =3 =1000 cubic centimetres =0.001 cubic metres, so 1 cubic metre =1000 litres. Small amounts of liquid are often measured in millilitres, where 1 millilitre =0.001 litres =1 cubic centimetre. Capacity is defined by the Oxford English Dictionary as the applied to the content of a vessel, and to liquids, grain, or the like. Capacity is not identical in meaning to volume, though closely related, Units of capacity are the SI litre and its derived units, and Imperial units such as gill, pint, gallon, and others. Units of volume are the cubes of units of length, in SI the units of volume and capacity are closely related, one litre is exactly 1 cubic decimetre, the capacity of a cube with a 10 cm side. In other systems the conversion is not trivial, the capacity of a fuel tank is rarely stated in cubic feet, for example. The density of an object is defined as the ratio of the mass to the volume, the inverse of density is specific volume which is defined as volume divided by mass. Specific volume is an important in thermodynamics where the volume of a working fluid is often an important parameter of a system being studied

24.
Mass
–
In physics, mass is a property of a physical body. It is the measure of a resistance to acceleration when a net force is applied. It also determines the strength of its gravitational attraction to other bodies. The basic SI unit of mass is the kilogram, Mass is not the same as weight, even though mass is often determined by measuring the objects weight using a spring scale, rather than comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity and this is because weight is a force, while mass is the property that determines the strength of this force. In Newtonian physics, mass can be generalized as the amount of matter in an object, however, at very high speeds, special relativity postulates that energy is an additional source of mass. Thus, any body having mass has an equivalent amount of energy. In addition, matter is a defined term in science. There are several distinct phenomena which can be used to measure mass, active gravitational mass measures the gravitational force exerted by an object. Passive gravitational mass measures the force exerted on an object in a known gravitational field. The mass of an object determines its acceleration in the presence of an applied force, according to Newtons second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A bodys mass also determines the degree to which it generates or is affected by a gravitational field and this is sometimes referred to as gravitational mass. The standard International System of Units unit of mass is the kilogram, the kilogram is 1000 grams, first defined in 1795 as one cubic decimeter of water at the melting point of ice. Then in 1889, the kilogram was redefined as the mass of the prototype kilogram. As of January 2013, there are proposals for redefining the kilogram yet again. In this context, the mass has units of eV/c2, the electronvolt and its multiples, such as the MeV, are commonly used in particle physics. The atomic mass unit is 1/12 of the mass of a carbon-12 atom, the atomic mass unit is convenient for expressing the masses of atoms and molecules. Outside the SI system, other units of mass include, the slug is an Imperial unit of mass, the pound is a unit of both mass and force, used mainly in the United States

25.
Moon mass
–
The Moon is an astronomical body that orbits planet Earth, being Earths only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, following Jupiters satellite Io, the Moon is second-densest satellite among those whose densities are known. The average distance of the Moon from the Earth is 384,400 km, the Moon is thought to have formed about 4.51 billion years ago, not long after Earth. It is the second-brightest regularly visible celestial object in Earths sky, after the Sun and its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its cycle of phases have made the Moon an important cultural influence since ancient times on language, calendars, art. The Moons gravitational influence produces the ocean tides, body tides, and this matching of apparent visual size will not continue in the far future. The Moons linear distance from Earth is currently increasing at a rate of 3.82 ±0.07 centimetres per year, since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft. The usual English proper name for Earths natural satellite is the Moon, the noun moon is derived from moone, which developed from mone, which is derived from Old English mōna, which ultimately stems from Proto-Germanic *mǣnōn, like all Germanic language cognates. Occasionally, the name Luna is used, in literature, especially science fiction, Luna is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of our moon. The principal modern English adjective pertaining to the Moon is lunar, a less common adjective is selenic, derived from the Ancient Greek Selene, from which is derived the prefix seleno-. Both the Greek Selene and the Roman goddess Diana were alternatively called Cynthia, the names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana is connected to dies meaning day, several mechanisms have been proposed for the Moons formation 4.51 billion years ago, and some 60 million years after the origin of the Solar System. These hypotheses also cannot account for the angular momentum of the Earth–Moon system. This hypothesis, although not perfect, perhaps best explains the evidence, eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists, You have eighteen months. Go back to your Apollo data, go back to computer, do whatever you have to. Dont come to our conference unless you have something to say about the Moons birth, at the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular. Afterward there were only two groups, the giant impact camp and the agnostics. Giant impacts are thought to have been common in the early Solar System, computer simulations of a giant impact have produced results that are consistent with the mass of the lunar core and the present angular momentum of the Earth–Moon system

26.
Density
–
The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume, ρ = m V, where ρ is the density, m is the mass, and V is the volume. In some cases, density is defined as its weight per unit volume. For a pure substance the density has the numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity, osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser. Thus a relative density less than one means that the floats in water. The density of a material varies with temperature and pressure and this variation is typically small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object, increasing the temperature of a substance decreases its density by increasing its volume. In most materials, heating the bottom of a results in convection of the heat from the bottom to the top. This causes it to rise relative to more dense unheated material, the reciprocal of the density of a substance is occasionally called its specific volume, a term sometimes used in thermodynamics. Density is a property in that increasing the amount of a substance does not increase its density. Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated easily and compared with the mass, upon this discovery, he leapt from his bath and ran naked through the streets shouting, Eureka. As a result, the term eureka entered common parlance and is used today to indicate a moment of enlightenment, the story first appeared in written form in Vitruvius books of architecture, two centuries after it supposedly took place. Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time, from the equation for density, mass density has units of mass divided by volume. As there are units of mass and volume covering many different magnitudes there are a large number of units for mass density in use. The SI unit of kilogram per metre and the cgs unit of gram per cubic centimetre are probably the most commonly used units for density.1,000 kg/m3 equals 1 g/cm3. In industry, other larger or smaller units of mass and or volume are often more practical, see below for a list of some of the most common units of density

27.
G-force
–
The g-force is a measurement of the type of acceleration that causes a perception of weight. Despite the name, it is incorrect to consider g-force a fundamental force, since g-force accelerations indirectly produce weight, any g-force can be described as a weight per unit mass. The types of forces involved are transmitted through objects by interior mechanical stresses, the g-force acceleration is the cause of an objects acceleration in relation to free-fall. The g-force acceleration experienced by an object is due to the sum of all non-gravitational. In practice, as noted, these are surface-contact forces between objects, such forces cause stresses and strains on objects, since they must be transmitted from an object surface. Because of these strains, large g-forces may be destructive, gravitation acting alone does not produce a g-force, even though g-forces are expressed in multiples of the acceleration of a standard gravity. Thus, the gravitational acceleration at the Earths surface produces g-force only indirectly. These mechanical forces actually produce the g-force acceleration on a mass, for example, the 1 g force on an object sitting on the Earths surface is caused by mechanical force exerted in the upward direction by the ground, keeping the object from going into free-fall. The upward contact-force from the ground ensures that an object at rest on the Earths surface is accelerating relative to the free-fall condition, stress inside the object is ensured from the fact that the ground contact forces are transmitted only from the point of contact with the ground. Objects allowed to free-fall in an inertial trajectory under the influence of only, feel no g-force acceleration. This is demonstrated by the conditions inside a freely falling elevator falling toward the Earths center. These are examples of coordinate acceleration without a sensation of weight, the experience of no g-force, however it is produced, is synonymous with weightlessness. An example here is a rocket in space, in which simple changes in velocity are produced by the engines. The unit of measure of acceleration in the International System of Units is m/s2, however, to distinguish acceleration relative to free-fall from simple acceleration, the unit g is often used. One g is the due to gravity at the Earths surface and is the standard gravity, defined as 9.80665 metres per second squared. Note that the definition does not vary with location—the g-force when standing on the moon is about 0.181 g. The unit g is not one of the SI units, which uses g for gram, also g should not be confused with G, which is the standard symbol for the gravitational constant. For example, it is said an F-16 fighter jet is able to sustain up to 9 Gs for a limited time

28.
Escape velocity
–
The escape velocity from Earth is about 11.186 km/s at the surface. More generally, escape velocity is the speed at which the sum of a kinetic energy. With escape velocity in a direction pointing away from the ground of a massive body, once escape velocity is achieved, no further impulse need be applied for it to continue in its escape. When given a speed V greater than the speed v e. In these equations atmospheric friction is not taken into account, escape velocity is only required to send a ballistic object on a trajectory that will allow the object to escape the gravity well of the mass M. The existence of escape velocity is a consequence of conservation of energy, by adding speed to the object it expands the possible places that can be reached until with enough energy they become infinite. For a given gravitational potential energy at a position, the escape velocity is the minimum speed an object without propulsion needs to be able to escape from the gravity. Escape velocity is actually a speed because it does not specify a direction, no matter what the direction of travel is, the simplest way of deriving the formula for escape velocity is to use conservation of energy. Imagine that a spaceship of mass m is at a distance r from the center of mass of the planet and its initial speed is equal to its escape velocity, v e. At its final state, it will be a distance away from the planet. The same result is obtained by a calculation, in which case the variable r represents the radial coordinate or reduced circumference of the Schwarzschild metric. All speeds and velocities measured with respect to the field, additionally, the escape velocity at a point in space is equal to the speed that an object would have if it started at rest from an infinite distance and was pulled by gravity to that point. In common usage, the point is on the surface of a planet or moon. On the surface of the Earth, the velocity is about 11.2 km/s. However, at 9,000 km altitude in space, it is less than 7.1 km/s. The escape velocity is independent of the mass of the escaping object and it does not matter if the mass is 1 kg or 1,000 kg, what differs is the amount of energy required. For an object of mass m the energy required to escape the Earths gravitational field is GMm / r, a related quantity is the specific orbital energy which is essentially the sum of the kinetic and potential energy divided by the mass. An object has reached escape velocity when the orbital energy is greater or equal to zero

29.
Temperature
–
A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer, several scales and units exist for measuring temperature, the most common being Celsius, Fahrenheit, and, especially in science, Kelvin. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, the kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic bodies, especially of fluids. Temperature is important in all fields of science including physics, geology, chemistry, atmospheric sciences, medicine. The Celsius scale is used for temperature measurements in most of the world. Because of the 100 degree interval, it is called a centigrade scale.15, the United States commonly uses the Fahrenheit scale, on which water freezes at 32°F and boils at 212°F at sea-level atmospheric pressure. Many scientific measurements use the Kelvin temperature scale, named in honor of the Scottish physicist who first defined it and it is a thermodynamic or absolute temperature scale. Its zero point, 0K, is defined to coincide with the coldest physically-possible temperature and its degrees are defined through thermodynamics. The temperature of zero occurs at 0K = −273. 15°C. For historical reasons, the triple point temperature of water is fixed at 273.16 units of the measurement increment, Temperature is one of the principal quantities in the study of thermodynamics. There is a variety of kinds of temperature scale and it may be convenient to classify them as empirically and theoretically based. Empirical temperature scales are historically older, while theoretically based scales arose in the middle of the nineteenth century, empirically based temperature scales rely directly on measurements of simple physical properties of materials. For example, the length of a column of mercury, confined in a capillary tube, is dependent largely on temperature. Such scales are only within convenient ranges of temperature. For example, above the point of mercury, a mercury-in-glass thermometer is impracticable. A material is of no use as a thermometer near one of its phase-change temperatures, in spite of these restrictions, most generally used practical thermometers are of the empirically based kind. Especially, it was used for calorimetry, which contributed greatly to the discovery of thermodynamics, nevertheless, empirical thermometry has serious drawbacks when judged as a basis for theoretical physics. Theoretically based temperature scales are based directly on theoretical arguments, especially those of thermodynamics, kinetic theory and they rely on theoretical properties of idealized devices and materials

30.
Kelvin
–
The kelvin is a unit of measure for temperature based upon an absolute scale. It is one of the seven units in the International System of Units and is assigned the unit symbol K. The kelvin is defined as the fraction 1⁄273.16 of the temperature of the triple point of water. In other words, it is defined such that the point of water is exactly 273.16 K. The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Lord Kelvin, unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or typeset as a degree. The kelvin is the unit of temperature measurement in the physical sciences, but is often used in conjunction with the Celsius degree. The definition implies that absolute zero is equivalent to −273.15 °C, Kelvin calculated that absolute zero was equivalent to −273 °C on the air thermometers of the time. This absolute scale is known today as the Kelvin thermodynamic temperature scale, when spelled out or spoken, the unit is pluralised using the same grammatical rules as for other SI units such as the volt or ohm. When reference is made to the Kelvin scale, the word kelvin—which is normally a noun—functions adjectivally to modify the noun scale and is capitalized, as with most other SI unit symbols there is a space between the numeric value and the kelvin symbol. Before the 13th CGPM in 1967–1968, the unit kelvin was called a degree and it was distinguished from the other scales with either the adjective suffix Kelvin or with absolute and its symbol was °K. The latter term, which was the official name from 1948 until 1954, was ambiguous since it could also be interpreted as referring to the Rankine scale. Before the 13th CGPM, the form was degrees absolute. The 13th CGPM changed the name to simply kelvin. Its measured value was 0.01028 °C with an uncertainty of 60 µK, the use of SI prefixed forms of the degree Celsius to express a temperature interval has not been widely adopted. In 2005 the CIPM embarked on a program to redefine the kelvin using a more experimentally rigorous methodology, the current definition as of 2016 is unsatisfactory for temperatures below 20 K and above 1300 K. In particular, the committee proposed redefining the kelvin such that Boltzmanns constant takes the exact value 1. 3806505×10−23 J/K, from a scientific point of view, this will link temperature to the rest of SI and result in a stable definition that is independent of any particular substance. From a practical point of view, the redefinition will pass unnoticed, the kelvin is often used in the measure of the colour temperature of light sources. Colour temperature is based upon the principle that a black body radiator emits light whose colour depends on the temperature of the radiator, black bodies with temperatures below about 4000 K appear reddish, whereas those above about 7500 K appear bluish

31.
Apparent magnitude
–
The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermore, the magnitude scale is logarithmic, a difference of one in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry, apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or often simply as V, the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the sky were said to be of first magnitude, whereas the faintest were of sixth magnitude. Each grade of magnitude was considered twice the brightness of the following grade and this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This implies that a star of magnitude m is 2.512 times as bright as a star of magnitude m +1 and this figure, the fifth root of 100, became known as Pogsons Ratio. The zero point of Pogsons scale was defined by assigning Polaris a magnitude of exactly 2. However, with the advent of infrared astronomy it was revealed that Vegas radiation includes an Infrared excess presumably due to a disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures, however, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, with the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30, astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The AB magnitude zeropoint is defined such that an objects AB, the dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√100 ≈2.512. Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of F2 F1 =100 Δ m 5 =100.4 Δ m ≈2.512 Δ m

32.
Angular diameter
–
The angular diameter or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view. In the vision sciences it is called the angle and in optics it is the angular aperture. The angular diameter can alternatively be thought of as the angle through which an eye or camera must rotate to look from one side of an apparent circle to the opposite side, Angular radius equals half the angular diameter. When D ≫ d, we have δ ≈ d / D, for practical use, the distinction is only significant for spherical objects that are relatively close, since the small-angle approximation holds for x ≪1, arcsin ⁡ x ≈ arctan ⁡ x ≈ x. Estimates of angular diameter may be obtained by holding the hand at right angles to an extended arm. In astronomy the sizes of objects in the sky are given in terms of their angular diameter as seen from Earth. Since these angular diameters are typically small, it is common to present them in arcseconds, an arcsecond is 1/3600th of one degree, and a radian is 180/ π degrees, so one radian equals 3600*180/ π arcseconds, which is about 206265 arcseconds. Therefore, the diameter of an object with physical diameter d at a distance D, expressed in arcseconds, is given by. These objects have a diameter of one arcsecond, an object of diameter 725. The angular diameter of the Sun, from a distance of one light-year, is 0. 03″, the angular diameter 0. 03″ of the Sun given above is approximately the same as that of a person at a distance of the diameter of the Earth. Thus the angular diameter of the Sun is about 250,000 times that of Sirius, the angular diameter of the Sun is also about 250,000 times that of Alpha Centauri A. The angular diameter of the Sun is about the same as that of the Moon, even though Pluto is physically larger than Ceres, when viewed from Earth Ceres has a much larger apparent size. While angular sizes measured in degrees are useful for larger patches of sky, we need much finer units when talking about the size of galaxies. The Moons motion across the sky can be measured in size, approximately 15 degrees every hour. A one-mile-long line painted on the face of the Moon would appear to us to be about one arc-second in length, in astronomy, it is typically difficult to directly measure the distance to an object. But the object may have a physical size and a measurable angular diameter. In that case, the angular diameter formula can be inverted to yield the Angular diameter distance to distant objects as d ≡2 D tan ⁡. In non-Euclidean space, such as our universe, the angular diameter distance is only one of several definitions of distance

433 Eros
–
433 Eros is an S-type near-Earth asteroid approximately 34. 4×11. 2×11.2 kilometres in size, the second-largest near-Earth asteroid after 1036 Ganymed. It was discovered in 1898 and was the first near-Earth asteroid discovered and it was the first asteroid orbited by an Earth probe. It belongs to the Amor group, Eros is a Mars-crosser asteroid, the

1.
Six different views of Eros in approximate natural color from NEAR-Shoemaker in February 2000

3.
Regolith of Eros, seen during NEAR's descent; area shown is about 12 meters (40 feet) across

4.
A composite image of the north polar region, with the craters Psyche above and Himeros below. The long ridge Hinks Dorsum, believed to be a thrust fault, can be seen snaking diagonally between them. The smaller crater in the foreground is Narcissus.

Dysnomia (moon)
–
Dysnomia —officially Eris I Dysnomia—is the only known moon of the dwarf planet Eris. Observations taken on 10 September revealed a moon in orbit around Eris, in keeping with the Xena nickname that was already in use for Eris, the moon was nicknamed Gabrielle by its discoverers, after Xenas sidekick. It is 60 times fainter than Eris in the K band a

1.
Dysnomia, to the left, and Eris, center (Hubble Space Telescope)

Hubble Space Telescope
–
The Hubble Space Telescope is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile, with a 2. 4-meter mirror, Hubbles four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubbles orbit o

1.
The Hubble Space Telescope as seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and final human spaceflight to it.

2.
Grinding of Hubble's primary mirror at Perkin-Elmer, March 1979

3.
The backup mirror, by Kodak; its inner support structure can be seen because it is not coated with a reflective surface.

4.
The OTA, metering truss, and secondary baffle are visible in this image of Hubble during early construction.

Michael E. Brown
–
Michael E. Brown is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology since 2003. His team has discovered many objects, notably the dwarf planet Eris. He is the author of How I Killed Pluto and Why It Had It Coming and he earned his A. B. in physics from Princeton University in 1987, whe

1.
Mike Brown

2.
Geography (features)

Chad Trujillo
–
Chadwick A. Chad Trujillo is an American astronomer, discoverer of minor planets and the co-discoverer of Eris, the most massive dwarf planet known in the Solar System. Trujillo works with software and has examined the orbits of the numerous trans-Neptunian objects. In late August 2005, it was announced that Trujillo, along with Michael E. Brown, a

Eris (mythology)
–
Eris is the Greek goddess of strife and discord. Her name is the equivalent of Latin Discordia, which means discord, Eris Greek opposite is Harmonia, whose Latin counterpart is Concordia. Homer equated her with the war-goddess Enyo, whose Roman counterpart is Bellona, the dwarf planet Eris is named after the goddess, as is the religion Discordianis

1.
Eris on an Attic plate, ca. 575–525 BC

2.
El Juicio de Paris by Enrique Simonet, 1904

3.
Golden apple of discord by Jakob Jordaens, 1633

4.
Das Urteil des Paris by Anton Raphael Mengs, c. 1757

Minor planet
–
A minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor exclusively classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, as of 2016, the orbits of 709,706 minor planets were archived at the Minor Planet Center,469,275 of which had received permanen

1.
Only very few minor planets are named. The vast majority is either numbered or still has a provisional designation (blue).

Dwarf planet
–
A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. The International Astronomical Union currently recognizes five dwarf planets, Ceres, Pluto, Haumea, Makemake, another hundred or so known objects in the Solar System are suspected to be dwarf planets. Individual astronomers recognize several of these, and in

Trans-Neptunian object
–
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune,30 astronomical units. Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, the first trans-Neptunian object to be discovered was Pluto in 1930. It took until 1992 to

1.
Asteroid belt

Plutoid
–
A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i. e. a body orbiting beyond Neptune that is massive enough to be rounded in shape. The term plutoid was adopted by the International Astronomical Union working group Committee on Small Bodies Nomenclature, the term plutoid is not widely used by astronomers, though ice dwarf is not uncommon.

1.
Asteroid belt

Scattered disc
–
The scattered disc is a distant circumstellar disc in the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40° and these extreme orbits are thought to be the result of gravita

1.
Eris, the largest known scattered-disc object (center), and its moon Dysnomia (left of object)

2.
Tempel 1, a Jupiter-family comet

3.
Asteroid belt

Minor planet moon
–
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, the first modern era mention of the possibility of an asteroid satellite was in connection with an occultation of the bright star Gamma Ceti by the minor planet Hebe in 1977.

1.
243 Ida and its moon Dactyl as imaged by Galileo in 1993

2.
Radar images of asteroid (66391) 1999 KW4 and its moon. The 'streaks' on the image are the moon's trail as it moved while the images were created.

Perihelion and aphelion
–
The perihelion is the point in the orbit of a celestial body where it is nearest to its orbital focus, generally a star. It is the opposite of aphelion, which is the point in the orbit where the body is farthest from its focus. The word perihelion stems from the Ancient Greek words peri, meaning around or surrounding, aphelion derives from the prep

1.
The perihelion and aphelion are the nearest and farthest points (apsides) of a body's direct orbit around the Sun.

Astronomical unit
–
The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick f

1.
Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here, the so-called " black drop effect ") and the rarity of observations.

2.
The red line indicates the Earth-Sun distance, which is on average about 1 astronomical unit.

Orbital eccentricity
–
The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is an orbit, values between 0 and 1 form an elliptical orbit,1 is a parabolic escape orbit. The term derives its name from the parameters of conic sections and it is normall

1.
Gravity Simulator plot of the changing orbital eccentricity of Mercury, Venus, Earth, and Mars over the next 50,000 years. The arrows indicate the different scales used. The 0 point on this plot is the year 2007.

Mean anomaly
–
In celestial mechanics, the mean anomaly is an angle used in calculating the position of a body in an elliptical orbit in the classical two-body problem. Define T as the time required for a body to complete one orbit. In time T, the radius vector sweeps out 2π radians or 360°. The average rate of sweep, n, is then n =2 π T or n =360 ∘ T, define τ a

1.
Area swept out per unit time by an object in an elliptical orbit (grey) and by an imaginary object in a circular orbit (red) which completes its orbit in the same period of time. Both sweep out equal areas in equal times, but the angular rate of sweep varies for the elliptical orbit and is constant for the circular orbit.

Degree (angle)
–
A degree, usually denoted by °, is a measurement of a plane angle, defined so that a full rotation is 360 degrees. It is not an SI unit, as the SI unit of measure is the radian. Because a full rotation equals 2π radians, one degree is equivalent to π/180 radians, the original motivation for choosing the degree as a unit of rotations and angles is u

1.
One degree (shown in red) and eighty nine (shown in blue)

Orbital inclination
–
Orbital inclination measures the tilt of an objects orbit around a celestial body. It is expressed as the angle between a plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth directly above the equator, the plane of the orbit is the same as the Earths equatorial plane. The general case is that

1.
Fig. 1: One view of inclination i (green) and other orbital parameters

Longitude of the ascending node
–
The longitude of the ascending node is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a direction, called the origin of longitude, to the direction of the ascending node. The ascending node is the point where the orbit of the passes through the plane of reference. Commonly used reference planes and

1.
The longitude of the ascending node.

Argument of periapsis
–
The argument of periapsis, symbolized as ω, is one of the orbital elements of an orbiting body. Parametrically, ω is the angle from the ascending node to its periapsis. For specific types of orbits, words such as perihelion, perigee, periastron, an argument of periapsis of 0° means that the orbiting body will be at its closest approach to the centr

1.
Fig. 1: Diagram of orbital elements, including the argument of periapsis (ω).

Natural satellite
–
A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 178 known natural satellites, four IAU-listed dwarf planets are also known to have natural satellites, Pluto, Haumea, Makemake, and Eris. As of January 2012, ov

1.
Nineteen natural satellites are large enough to be round, and one, Saturn 's moon, Titan, has a substantial atmosphere.

2.
Two moons: Saturn's natural satellite Dione occults Enceladus

3.
Artist impression of Rhea 's proposed rings

4.
Discovery image of Styx, taken by Hubble’s WFC3 in 2012

Spheroid
–
A spheroid, or ellipsoid of revolution, is a quadric surface obtained by rotating an ellipse about one of its principal axes, in other words, an ellipsoid with two equal semi-diameters. If the ellipse is rotated about its axis, the result is a prolate spheroid. If the ellipse is rotated about its axis, the result is an oblate spheroid. If the gener

1.
oblate spheroid

Volume
–
Volume is the quantity of three-dimensional space enclosed by a closed surface, for example, the space that a substance or shape occupies or contains. Volume is often quantified numerically using the SI derived unit, the cubic metre, three dimensional mathematical shapes are also assigned volumes. Volumes of some simple shapes, such as regular, str

1.
A measuring cup can be used to measure volumes of liquids. This cup measures volume in units of cups, fluid ounces, and millilitres.

Mass
–
In physics, mass is a property of a physical body. It is the measure of a resistance to acceleration when a net force is applied. It also determines the strength of its gravitational attraction to other bodies. The basic SI unit of mass is the kilogram, Mass is not the same as weight, even though mass is often determined by measuring the objects we

1.
Depiction of early balance scales in the Papyrus of Hunefer (dated to the 19th dynasty, ca. 1285 BC). The scene shows Anubis weighing the heart of Hunefer.

2.
The kilogram is one of the seven SI base units and one of three which is defined ad hoc (i.e. without reference to another base unit).

3.
Galileo Galilei (1636)

4.
Distance traveled by a freely falling ball is proportional to the square of the elapsed time

Moon mass
–
The Moon is an astronomical body that orbits planet Earth, being Earths only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, following Jupiters satellite Io, the Moon is second-densest satellite among those whose densities are known. The average distance of the Moon from the Earth is 384,400 km, the Moon

1.
Full moon as seen from Earth's northern hemisphere

2.
The Moon, tinted reddish, during a lunar eclipse

3.
Near side of the Moon

4.
Far side of the Moon

Density
–
The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume, ρ = m V, where ρ is the density, m is the mass, and V is the volume. In some cases, density

1.
Air density vs. temperature

G-force
–
The g-force is a measurement of the type of acceleration that causes a perception of weight. Despite the name, it is incorrect to consider g-force a fundamental force, since g-force accelerations indirectly produce weight, any g-force can be described as a weight per unit mass. The types of forces involved are transmitted through objects by interio

1.
This top-fuel dragster can accelerate from zero to 160 kilometres per hour (99 mph) in 0.86 seconds. This is a horizontal acceleration of 5.3 g. Combined with the vertical g-force in the stationary case the Pythagorean theorem yields a g force of 5.4 g.

2.
In straight and level flight, lift (L) equals weight (W). In a banked turn of 60°, lift equals double the weight (L=2W). The pilot experiences 2 g and a doubled weight. The steeper the bank, the greater the G-Forces.

3.
This acrobatic airplane is pulling up in a +g maneuver; the pilot is experiencing several g's of inertial acceleration in addition to the force of gravity. The cumulative vertical axis forces acting upon his body make him momentarily 'weigh' many times more than normal.

4.
John Stapp was subjected to 15 g for 0.6 seconds and a peak of 22 g during a 19 March 1954 rocket sled test. He would eventually survive a peak of more than 46 g, with more than 25 g for 1.1 seconds.

Escape velocity
–
The escape velocity from Earth is about 11.186 km/s at the surface. More generally, escape velocity is the speed at which the sum of a kinetic energy. With escape velocity in a direction pointing away from the ground of a massive body, once escape velocity is achieved, no further impulse need be applied for it to continue in its escape. When given

1.
Luna 1, launched in 1959, was the first man-made object to attain escape velocity from Earth (see below table).

2.
General

Temperature
–
A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer, several scales and units exist for measuring temperature, the most common being Celsius, Fahrenheit, and, especially in science, Kelvin. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, the kinetic theory offer

1.
Annual mean temperature around the world

2.
Body temperature variation

3.
A typical Celsius thermometer measures a winter day temperature of -17 °C.

4.
Plots of pressure vs temperature for three different gas samples extrapolated to absolute zero.

Kelvin
–
The kelvin is a unit of measure for temperature based upon an absolute scale. It is one of the seven units in the International System of Units and is assigned the unit symbol K. The kelvin is defined as the fraction 1⁄273.16 of the temperature of the triple point of water. In other words, it is defined such that the point of water is exactly 273.

Apparent magnitude
–
The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermor

1.
Asteroid 65 Cybele and two stars, with their magnitudes labeled

2.
30 Doradus image taken by ESO 's VISTA. This nebula has an apparent magnitude of 8.

Angular diameter
–
The angular diameter or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view. In the vision sciences it is called the angle and in optics it is the angular aperture. The angular diameter can alternatively be thought of as the angle through which an eye or camera must rotate to look from

1.
Comparison of angular diameter of the Sun, Moon, planets and the International Space Station. To get a true representation of the sizes, view the image at a distance of 103 times the width of the "Moon: max." circle. For example, if this circle is 10 cm wide on your monitor, view it from 10.3 m away.

1.
The original proposal would have immediately added three planets, shown here in a size comparison to Earth. Leftmost is Pluto (shown in lieu of Eris), then Charon, Ceres, and Earth

2.
Euler diagram showing the types of bodies in the Solar System.

3.
The twelve "candidate planets" that were possibilities for inclusion under the originally proposed definition. Note that all but the last three are trans-Neptunian objects. The smallest three (Vesta, Pallas, Hygeia) are in the asteroid belt.

1.
One of the many early concepts for a mission to Pluto was to send a Mariner Mark II spacecraft. The idea would later be ruled out in favour of a smaller, less expensive spacecraft similar to the Pluto 350 concept.

2.
The New Horizons space probe

3.
The infamous "Not Yet Explored" USPS stamp that inspired scientists to research into and lobby for a mission to Pluto.

4.
Concept art for the Pluto Kuiper Express; the last iteration of the original Pluto mission concepts, eventually cancelled in 2000.

1.
The 77 cm Schmidt-telescope from 1966 at Brorfelde Observatory was originally equipped with photographic film, and an engineer is here showing the film-box, which was then placed behind the locker at the center of the telescope (at the telescope's prime focus)

2.
Optical ray paths inside a Schmidt camera

3.
The 2 meter diameter (Alfred-Jensch -Telescope at the Karl Schwarzschild Observatory in Tautenburg, Thuringia, Germany is the largest Schmidt camera in the world.

4.
One of the Baker - Nunn cameras used by the Smithsonian satellite-tracking program.

1.
Comparison of angular diameter of the Sun, Moon, planets and the International Space Station. To get a true representation of the sizes, view the image at a distance of 103 times the width of the "Moon: max." circle. For example, if this circle is 10 cm wide on your monitor, view it from 10.3 m away.

4.
Conic sections describe the possible orbits (yellow) of small objects around the earth. A projection of these orbits onto the gravitational potential (blue) of the earth makes it possible to determine the orbital energy at each point in space.

3.
Prometheus (1868 by Gustave Moreau). The myth of Prometheus first was attested by Hesiod and then constituted the basis for a tragic trilogy of plays, possibly by Aeschylus, consisting of Prometheus Bound, Prometheus Unbound, and Prometheus Pyrphoros.

4.
The Roman poet Virgil, here depicted in the fifth-century manuscript, the Vergilius Romanus, preserved details of Greek mythology in many of his writings.

1.
Rama (right) seated on the shoulders of Hanuman, battles the demon-king Ravana, scene from Ramayana.

2.
The Creation of the Cosmic Ocean and the Elements, folio from the Shiva Purana, c. 1828.

3.
Vishnu is known as the preserver aspect of Brahmin. In his four hand he carries at all times, a lotus, a conch shell, a club, and a discus.The ten avatars of Vishnu, (Clockwise, from Left upper corner) Matsya, Kurma, Varaha, Vamana, Krishna, Kalki, Buddha, Parshurama, Rama and Narasimha, (in centre) Krishna

4.
An illustration of the family of Shiva, consisting of Shiva, Parvati, Ganesha and Murugan

1.
A futuristic setting is a common but not a necessary hallmark of science fiction. A common thread in science fiction is exploring the potential consequences of scientific and other innovations on people's lives.

1.
Romulus and Remus, the Lupercal, Father Tiber, and the Palatine on a relief from a pedestal dating to the reign of Trajan (AD 98–117)

2.
In this wall painting from Pompeii, Venus looks on while the physician Iapyx tends to the wound of her son, Aeneas; the tearful boy is her grandson Ascanius, also known as Iulus, legendary ancestor [citation needed] of Julius Caesar and the Julio-Claudian dynasty